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Message-ID: <3bcdb026-8558-43ca-80c1-776216dcd86c@igalia.com>
Date: Thu, 10 Aug 2023 14:01:20 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com,
dvhart@...radead.org, dave@...olabs.net, tglx@...utronix.de,
axboe@...nel.dk, Andrew Morton <akpm@...ux-foundation.org>,
urezki@...il.com, hch@...radead.org, lstoakes@...il.com,
Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
linux-mm@...ck.org, linux-arch@...r.kernel.org,
malteskarupke@....de, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v2 05/14] futex: Add sys_futex_wake()
Em 10/08/2023 09:13, Peter Zijlstra escreveu:
> On Wed, Aug 09, 2023 at 07:25:19PM -0300, André Almeida wrote:
>> Hi Peter,
>>
>> Em 07/08/2023 09:18, Peter Zijlstra escreveu:
>>> To complement sys_futex_waitv() add sys_futex_wake(). This syscall
>>> implements what was previously known as FUTEX_WAKE_BITSET except it
>>> uses 'unsigned long' for the bitmask and takes FUTEX2 flags.
>>>
>>> The 'unsigned long' allows FUTEX2_SIZE_U64 on 64bit platforms.
>>>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> Acked-by: Geert Uytterhoeven <geert@...ux-m68k.org>
>>> ---
>>
>> [...]
>>
>>> +/*
>>> + * sys_futex_wake - Wake a number of futexes
>>> + * @uaddr: Address of the futex(es) to wake
>>> + * @mask: bitmask
>>> + * @nr: Number of the futexes to wake
>>> + * @flags: FUTEX2 flags
>>> + *
>>> + * Identical to the traditional FUTEX_WAKE_BITSET op, except it is part of the
>>> + * futex2 family of calls.
>>> + */
>>> +
>>> +SYSCALL_DEFINE4(futex_wake,
>>> + void __user *, uaddr,
>>> + unsigned long, mask,
>>> + int, nr,
>>> + unsigned int, flags)
>>> +{
>>
>> Do you think we could have a
>>
>> if (!nr)
>> return 0;
>>
>> here? Otherwise, calling futex_wake(&f, 0, flags) will wake 1 futex (if
>> available), which is a strange undocumented behavior in my opinion.
>
> Oh 'cute' that.. yeah, but how about I put it ...
>
>>> + if (flags & ~FUTEX2_VALID_MASK)
>>> + return -EINVAL;
>>> +
>>> + flags = futex2_to_flags(flags);
>>> + if (!futex_flags_valid(flags))
>>> + return -EINVAL;
>>> +
>>> + if (!futex_validate_input(flags, mask))
>>> + return -EINVAL;
>
> here, because otherwise we get:
>
> sys_futex_wake(&f, 0xFFFF, 0, FUTEX2_SIZE_U8)
>
> to return 0, even though that is 'obviously' nonsensical and should
> return -EINVAL. Or even garbage flags would be 'accepted'.
>
> (because 0xFFFF is larger than U8 can accomodate)
>
That make sense to me, but we would also want to validate the value of
f, if it's NULL or something strange to return -EINVAL... but this
happens only inside get_futex_key()...
To make this right, I think we would need to move this verification to
the syscall validation part:
if (unlikely((address % sizeof(u32)) != 0))
return -EINVAL;
if (unlikely(!access_ok(uaddr, sizeof(u32))))
return -EFAULT;
And have u32 replaced with the proper size being used.
>>> +
>>> + return futex_wake(uaddr, flags, nr, mask);
>>> +}
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